1. Field of the Invention
The present invention relates to a blade mounting device in a cutting tool.
2. Description of the Prior Art
In a conventional reciprocating cutting tool such as a jig saw and a reciprocating saw, a blade is frequently changed in accordance with the thickness and other factors of a work to be cut. If hand tools such as spanners or screwdrivers are required for changing the blade, much time is required to operate such hand tools. Thus, the operation of such hand tools is troublesome. In addition, the operation of such hand tools involves a problem that the hand tools are liable to be lost. For this reason, this kind of cutting tool should preferably require no hand tools for changing the blade. In order to solve this problem, a blade mounting device has been proposed in U.S. Pat. No. 5,165,173.
The construction of the above U.S. patent will now be explained with reference to FIGS. 11 and 12.
As shown in FIG. 11 which corresponds to FIG. 2 of the U.S. patent, an end cap 107 is secured to one end of a drive shaft 103 which is reciprocally driven in an axial direction. A pair of diametrically opposed entry slots 108 and a pair of diametrically opposed retention grooves 109 are formed in the end cap 107. The retention grooves 109 are circumferentially displaced by an angle of 90.degree. from the entry slots 108. A locking member 113 is disposed inside of the drive shaft 103 and is vertically movable relative to the drive shaft 103. The locking member 113 is normally biased in a downward direction by a compression spring 114. A slot 112 is formed in the locking member 113 for permitting insertion of a part of a blade B between bayonet lugs Ba and an upper end Bt of the blade B. When the upper end Bt of the blade B is brought to abut on the locking member 113 at the upper end of the slot 112, the downward biasing force of the compression spring 114 is applied to the blade B.
With this construction, the blade B is inserted into the end cap 107 with the bayonet lugs Ba being in alignment with the entry slots 108, and the blade B is thereafter rotated by an angle of 90.degree. so as to bring the bayonet lugs Ba in alignment with the retention grooves 109. Then, the blade B is pressed downwardly by the compression spring 114 and the bayonet lugs Ba are brought to engage the retention grooves 109 so as to be retained therein. Thus, the blade B is mounted on the drive shaft 103 not to be extracted downwardly therefrom. Here, a pair of outwardly extending lugs 113a are formed on the lower end of the locking member 113. In the mounting state of the blade B as described above, the lugs 113a are brought to engage the entry slots 108 so as to prevent the blade B as well as the locking member 113 from rotation.
In order to remove the blade B from the drive shaft 103, the locking member 13 is lifted by moving a lever to an unlocking position. The lever is connected to the locking member 113 via a tension member 118. Thus, the lugs 113a are disengaged from the entry slots 108 against the biasing force of the compression spring 114, and in this state, the blade B can be rotated. The blade B is then rotated by an angle of 90.degree. to disengage the bayonet lugs Ba from the retention grooves 109 and to bring the bayonet lugs Ba in alignment with the entry slots 108, so that the blade B can be removed.
After removing the blade B, another blade B is inserted into the end cap 107 and is rotated by an angle of 90.degree., so that its bayonet lugs Ba are brought into alignment with the retention grooves 109. When the lever is moved to a locking position, the blade B is pressed downwardly by the compression spring 114 via the locking member 113, so that the bayonet lugs Ba are brought to engage the retention grooves 109. Another blade B can be thus mounted on the drive shaft 103.
In order to hold the bayonet lugs Ba of the blade B in the engaging position with the retention recesses 109, the U.S. patent proposes an alternative construction as shown in FIG. 12 which corresponds to FIG. 5 of the U.S. patent.
In the embodiment shown in FIG. 12, a lock screw 128 is inserted into the drive shaft 103 and is in engagement with a female thread formed on an inner peripheral surface of the drive shaft 103, so that the upper portion of the blade B can be pressed or released by the axial movement of the lock screw 128. The screw 128 has an extension 129 extending upwardly from the screw 128 and having a semi-circular configuration in section. A screw rotating member 130 is also inserted into the drive shaft 103 and has a semi-circular configuration in section similar to the extension 129. The extension 129 and the screw rotating member 130 have confronting and mutually sliding flat surfaces. The screw rotating member 130 however is not threadably engaged with the drive shaft 103. Therefore, when the screw rotating member 130 is rotated by means of an operation knob 137, the extension 129 is rotated together with the screw rotating member 130. The screw 128 is then rotated together with the extension 129, so that it is moved to press the upper end Bt of the blade B or to release the same. Although the screw rotating member 130 is rotated, it does not move in an axial direction. Therefore, only the extension 129 as well as the screw 128 is moved in the axial direction with its flat end slidably abutting on the flat surface of the extension 129.
Although the above blade mounting device of the U.S. patent permits the blade B to be mounted and removed without using hand tools, the device can only be applicable to a blade B having the bayonet lugs Ba. Therefore, this device is inconvenient in that the device cannot be applied to blades of different types.
More specifically, other than the blade having bayonet lugs which are adapted to be engaged by the drive shaft as described above (hereinafter called "lug-type blade"), the prior art has used a blade having a through-hole formed on its upper end for inserting a protrusion which serves to prevent the blade from removal (hereinafter called "hole-type blade"). The blade of the latter type cannot be mounted on the device of the U.S. patent since it does not include any parts for engagement with the retention grooves 109.
In addition, with the device of the U.S. patent, the mounting operation of the blade is very troublesome since it requires to rotate the blade B by an angle of 90.degree. so as to bring the bayonet lugs Ba into alignment with the retention grooves 109 for engagement therewith after the blade B has been inserted into the end cap 107.
Further, with the construction shown in FIG. 12 where the blade B is fixed in position by the lock screw 128 which is rotated to press the upper portion of the blade B, since the lock screw 128 is axially moved by a distance of its one pitch through each one rotation of the operation knob 137, the operation knob 137 must be rotated several times for moving the lock screw 128 by a distance corresponding to several pitches which is sufficient for changing the blade B. Therefore, this construction involves the problem that much time is required for changing the blade. Other than the mounting device as described above, this problem is also caused in a mounting device in which a lock screw for fixing the blade is rotated by a hand tool such as a screwdriver.
The amount of movement of the lock screw 128 for one rotation of the operation knob 137 can be increased by merely increasing pitch of the lock screw 128, and the above problem can be solved to some extent. However, as the pitch increases, the lock screw 128 tends to be loosened by vibration and other external forces, so that the mounting state of the blade B becomes unstable. Therefore, this solution is not desirable.